1 Ch 13 Culverts

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OFFICE OF BRIDGE DEVELOPMENT

MANUAL FOR HYDROLOGIC AND HYDRAULIC

DESIGN

CHAPTER 13 CULVERTS

Guidelines for the Selection and Design

of Culvert Installations

This Chapter remains in draft form pending assessment of the guidance presented

For accommodating bankfull flows

DRAFTSEPTEMBER 2006

PREFACE


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Please note that this manual is in final draft form. The design concept of using a combination of

main channel and flood plain culverts is not standard practice, but is considered by the SHA on a

case by case basis.

Incomplete items in this manual include the preparation of Figures and the development of amethod for evaluating the potential for silting of culverts. The method in Appendix B is used by

some hydraulic engineers to estimate this condition. It is retained in this chapter for

consideration until such time as an updated method developed for Maryland streams is

completed.

Chapter 13


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Guidelines for the Selection and Design of Culvert Installations

Table of Contents

13.1 Introduction 4

13.2 Policy..5

13.3 Design Objectives7

13.4 Location Studies13.4.1 Location Studies.....9.13.4.2 Highway Geometry and Stream Plan Form.. 9

13.4.3 Hydrology Studies. 913.4.4 Stream Inventory and Classification ..1013.4.5 Selection of Structure Type (Culvert vs Bridge) 11

13.5 Culvert Design Procedures13.5.1Introduction...13 13.5.2 Culvert Alternatives .1413.5.3 Water Surface Profiles..1413.5.4 Upstream Transition Section ...1413.5.5 Main Channel Culvert ..1613.5.6 Flood Plain Culverts.1713.5.7 Culvert Headwall and Endwall .1913.5.8 Hydraulic Analysis20

13.5.9 Culvert Outlets..20

13.6 Culvert Design Procedures - Special Design Considerations

13.6.1 Type A Streams.2313.6.2 Type DA Streams......2313.6.3 Type F Streams....2413.6.4 Type G Streams ...2513.6.5 Stream Crossings at Bends, Pools and Confluences 26

13.7 Applications Case Histories 29

13.8 Outlet Velocities, Degradation and Scour

30

13.9 Silting of Culverts .32


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Chapter 13

Guidelines for the Selection and Design of Culvert Installations

Table of Contents

13.10 Effects of Ice and Debris 35

13.11 Structural

Requirements...36

13.12 Durability Requirements 37

13.13 Traffic Safety Requirements 39

13.14 ConstructionDetails.40

13.15 References .. 41

Appendix A Procedures for the Hydraulic Design of Culverts

Appendix B Procedure for Evaluating the Potential for Silting up of Culverts


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13.1 Introduction________________________________________________

Various definitions for a culvert are used by the Office of Bridge Development, depending upon

the purpose of the definition:

Any structure designed hydraulically as a culvert, regardless of size, is considered as aculvert for purposes of this chapter Any structure that has a paved bottom is defined by the Maryland Department of the

Environment as a culvert, and this definition is used by SHA in preparing permit

applications.

Structures 20 feet or wider in centerline length between extreme ends of openings areconsidered as bridges for purpose of the bridge inventory. These bridges may include

structures designed as culverts and are therefore treated as culverts for purposes of this

chapter.

Small structures are defined as structures from 5 feet up to 20 feet in centerline lengthbetween extreme ends of openings. These may be either bridges or culverts.

Hydraulic design procedures for culverts are based on the publications and software of the

Federal Highway Administration. These procedures are discussed in Appendix A

The design of a highway stream crossing involves consideration and resolution of a number of

issues and concerns in order to achieve the design objectives for the site. The designer should list

and prioritize these objectives at the beginning of the location/design process. This exercise will

provide an opportunity to evaluate the type of design best suited for a particular location. It will

also serve to highlight conflicts between objectives for a given alternative design.

These design objectives, listed in Table 1 of Section 13.2, can be categorized in the following

manner:

1. Objectives regarding the structure itself such as its structural, geotechnical and hydraulicperformance and the overall effect of the design on safety, maintenance and traffic

operations. This includes minimizing any effects of the stream on the structure, the highway

and related flood hazards.

2. Objectives relating to minimizing any adverse effects of the structure on the stream, its floodplain, wetlands and associated habitat.

This chapter provides an approach to the selection and design of culvert installations based on


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13.1 Introduction________________________________________________

consideration of the relative significance and priorities of the objectives in these categories.

Objectives in both categories need to be satisfied to the extent practicable, in order to achieve a

successful, cost-effective design.

This guideline addresses the collection, evaluation and application of information for culvert

design. It is not intended for use as a step by step procedure, applicable under all conditions.

Each location will have special features that will need to be evaluated and dealt with on the basis

of engineering judgment.

The Group 1 objectives in Table 1 are determined by the SHA engineers in accordance with

current state and federal regulations, SHA policies and design criteria. These engineering

considerations are not normally reviewed by others except with regard to compliance with State

and Federal flood plain regulations.

The Group 2 objectives in Table 1, as well as the objectives common to both groups, areaddressed by SHA in consultation with representatives of other regulatory and environmental

agencies. Designers and reviewers need to reach a common understanding early in the project

development process with regard to what the objectives should be and what approaches should

be considered in achieving these objectives.

The SHA has completed a study of Maryland streams in cooperation with the U. S. Fish and

Wildlife Service, the Environmental Protection Agency, the U. S. Geological Survey, the

Maryland Department of Environment Protection and other State and Federal agencies. The

purpose of this study is to identify, inventory and classify the hydrologic, hydraulic and

geomorphological characteristics of the streams in the various hydrologic provinces in Maryland.

The recommended guideline for the evaluation of stream morphology is the text entitled

Applied Stream Morphology by Dave Rosgen (Reference 7). This reference serves as a

companion text to the guidelines set forth in this chapter. The information from the Maryland

Stream Study is being put to use to improve field investigation procedures and to facilitate

achievement of the design objectives in Table 1.

Additional information regarding general design policies and procedures affecting culverts is set

forth in the following chapters of the SHA Manual for Hydrologic and Hydraulic Design:

Chapter 3 - Policy

Chapter 8 - HydrologyChapter 9 - Channels

Chapter 10 - Bridges

Chapter 11 - Scour

Chapter 14 - Stream Morphology


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13.2 Policy

The following policies of the Office of Bridge Development apply specifically to culverts.

Culverts shall be located and designed to present a minimum hazard to traffic and people. All culverts shall be hydraulically designed. The design flood and overtopping flood shall be selected in accordance with the criteria set

forth in Chapter 10, Bridges.

The culvert installation shall be designed, to the extent practicable, to meet the objectives inthis chapter regarding stream stability, passage of fish and wildlife and other environmental

concerns.

Any culvert founded on footings shall be evaluated for resistance to scour in accordance withChapter 11.

Material selection shall include consideration of service life and the effects of corrosion andabrasion.

The detail of documentation for each culvert site shall be commensurate with the risk andimportance of the structure and the site conditions. Design data and calculations shall be

assembled in an orderly fashion and retained for future reference as provided for in Chapter

5, Documentation. A Hydrology and Hydraulics Summary Sheet shall be completed for each

culvert and included with the PS&E plans for the project (Chapter 4, Documentation).

Consideration shall be given to the potential problems of debris and silting up of culvertbarrels.

Where practicable, means shall be provided for personnel and equipment access to facilitatemaintenance (Chapter 7).

Culverts shall be regularly inspected and maintained (Chapter 7).These policies, along with design guidance used to implement the policies are discussed at

greater length in the following sections of this chapter and in other chapters of the Manual.


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13.3 Design Objectives____________________________________________

Table 1 below presents typical objectives for culvert design.

Table 1 - Primary Objectives to be Achieved in Culvert Design

Group 1 Objectives

Objectives relating to the structure itself such as its structural, geotechnical and hydraulic

design and the overall effect of the design on safety, maintenance and traffic operations. This

includes minimizing any effects of the stream on the structure, the highway and related flood

hazards.

1. Provide for the safety of the public; structure to remain stable for worst-case floodconditions.

2. Provide a cost-effective and maintainable design3. Meet State requirements for strength and durability4. Minimize potential for uplift, piping and pressure forces on culverts5. Minimize maintenance problems with inorganic and organic debris6. *Meet requirements for flood plain management7. * Maintain the natural stream channel stability of dimension, pattern and profile so that over

time the channel features are maintained and the channel neither aggrades nor degrades.

(Select culvert installation to maintain bankfull geometry of width and depth).

8. *Control scour, erosion and degradation of bed and banks at culvert inlet and outlet and indownstream channel; control deposition of material in culvert barrel.

*These objectives can be considered common to both Group 1 and Group 2

Group 2 Objectives

Objectives relating to minimizing the effect of the structure on the stream, its flood plain,

wetlands and associated habitat.

1. Provide for passage of fish and wildlife

2. Maintain and enhance fish and wildlife habitat

3. Create an aesthetic design

4. See objectives 6-8 above.


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13.3 Design Objectives____________________________________________

The primary objective of the SHA is to achieve a safe, stable stream crossing that is

commensurate with the risks posed to the travelling public at the site. The design criteria for use

in considering the design flood and the risks associated with a proposed design are set forth in

Chapters 8, 9 and 10 and will not be repeated here. The stream stability and other environmentalobjectives will be addressed in later sections of this chapter.

This guide introduces the concept of using a combination of main channel and flood plain

culverts, when practicable, to accomplish the design objectives in Table 1 However, it is

recognized that there are thousands of culverts under Maryland roads today designed as a single

box or pipe that perform in a manner that substantially meets the design objectives in Table 1. It

is expected that designers will continue to encounter stream crossings where site conditions

permit the use of a single culvert to satisfy the design objectives. Where a culvert installation

without flood plain culverts is proposed, it becomes especially important to review the proposed

design in the light of the design objectives. In particular consider the flow patterns, which result

from the collection of main channel and flood plain flows and their subsequent discharge into thedownstream channel.

Some of the site features that may favor a main channel culvert without flood plain culverts are

noted below. However, the need for flood plain culverts should be determined on a case by case

basis:

crossings of smaller streams, particularly ephemeral streams, replacement of an existing main channel culvert at a location where the stream is stable and

has successfully adjusted to the conditions established by the existing (main channel) culvert.

short culvert installations as is typical for low volume road stream crossings, locations where provision for fish passage is not required (ephemeral streams), crossings of streams where most of the flow for design conditions is in the channel, and

where the flood plain is small or conveys little flow (Type B channels, for example),

crossings where a large waterway opening is desired to accommodate passage of debriscarried by the channel, and where provisions are made to design and maintain a stable

channel section (This may be a bridge vs. culvert issue.).


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13.4 Location Studies_______________________________

13.4.1 Location Studies

Office of Bridge Development engineers are to be represented on study groups convened by the

Office of Planning and Preliminary Engineering to assure that highway stream crossings arelocated, to the extent practicable, at hydraulically favorable locations (See Chapter 5).

Preliminary field and office studies and evaluations are made of the location of proposed culvert

installations early in the project development process so as to enable the Office of Bridge

Development to make informed decisions regarding the proposed crossing locations and the

types of structures to be built.

These preliminary studies normally include consideration of the proposed line, grade and typical

section of the highway, the hydrology of the watershed and the geomorphology of the stream and

its flood plain. The extent of such location studies should be dependent upon the importance of

the proposed structures and the flood hazards and environmental concerns involved with the site.These considerations and concerns are outlined below.

13.4.2 Highway Geometry and Stream Plan Form

Favorable crossing locations (Figure 1) are those that:

are located on a relatively straight and stable section of the stream, typically a riffle, provide for a highway alignment generally normal to the flow in the channel and flood plain,

avoiding locations of sharp bends, severe meanders, confluences or other areas of converging

or diverging flow patterns and associated turbulence. Culvert skew should not exceed 450.

are maintaining stable stream conditions in the reach of the stream affected by the highwaycrossing.

It may not be feasible to obtain an optimum highway alignment for every stream crossing;

nevertheless, it is important that efforts be made at this early stage of project development to

select the structure location and type that best fits the particular site conditions.

13.4.3 Hydrology Studies

Hydrology studies shall be performed in accordance with the policies and procedures set forth in

Chapter 8. These studies provide the information to develop a flood frequency plot for the

crossing site for flows from the bankfull stage (approximately 1.5-year flood for natural

channels) to the 500-year flood. Hydrology studies should be initiated early in the project


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development process so that information on flood flows will be available for consideration when

decisions are made regarding the location of the highway stream crossing and the selection of

structure type. Chapter 5 discusses the project development process for projects involving

structure designed by the Office of Bridge Development. A hydrology report shall be preparedsummarizing appropriate information in accordance with the guidance set forth in Chapters 3,

Policy and Chapter 8, Hydrology. SHA and the Maryland Department of the Environment

(MDE) have agreed upon the method to be used in making estimates of flood peaks (17).

In Maryland, State regulations require thatflooddischarges be estimated on the basis of ultimate

development in the watershed as determined from current zoning maps. Preliminary estimates of

the magnitude ofbankfull flows can be based on the regional curves set forth in the Maryland

Stream Study report prepared by the U.S. Fish and Wildlife Service (15). These estimates are to

be verified by field measurements made during the stream stability study. For sites with gaging

stations, the frequency of the bankfull flow can be estimated from the station records. For

ungaged sites, the bankfull flow should be plotted on the flood-frequency plot developed for thestream, using a return period between one and two years. See also Reference (15).

13.4.4 Stream Inventory and Classification

The protocols developed by the Fish and Wildlife Service (Reference 15) are to be used for

measurements and inventories of stream characteristics. The stream should be classified in

accordance with the Rosgen classification system (References 7 and 8). The stream stability and

geomorphology study should address the following considerations:

Stream and flood plain characteristics at the proposed highway crossing, and when necessarystream and flood plain characteristics at a reference reach determined to be a stable reach thatis typical of the stream morphology in the vicinity of the highway crossing.

Cross-sections of the channel and flood plain to provide necessary input for the preparationof water surface profiles using HEC-RAS. Note that if a bottomless culvert alternative is

to be considered, cross-section elevations should be taken high enough to encompass the

500-year flood.

A preliminary assessment regarding the stability of the stream bed and banks, adverse effectsof urbanization, actual or potential aggradation, degradation or lateral migration, etc.

Preparation of a Stream Inventory and Classification Report to summarize the results of thefield and office studies. The report should follow the methodology set forth in Attachment

A. of Reference 15. Particular attention is directed towards obtaining measurements of

bankfull width and depth. The bankfull depth elevations and channel bank elevations need to

be plotted on the stream profile and cross-sections in the reach of the stream under study.


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The process described above provides for an initial classification of the stream for use in

assessing the utility of a proposed culvert design. The designer will need to make an evaluation

of the classification assigned to the stream as well as the extent to which the stream classification

is used in the design the culvert. This will be particularly true in cases where the stream exhibitscharacteristics of more than one stream type or where the stream has been disturbed by changes

to the channel or its watershed. Streams undergoing evolutionary change will need special

consideration to decide how best to accommodate the potential stream type shifts which may

occur over the life of the highway structure.

The general guidance and observations provided in the various sections of this chapter (for a C-

type stream or a G-type stream, etc.) must be independently evaluated to verify that the guidance

is appropriate for the given site conditions. Each stream crossing is unique. It cannot be assumed

that the design approach for two different streams with the same stream classification should

always be exactly the same just because the streams share the same classification of stream type.

Notwithstanding the cautions noted above, the concepts of (1) using the Rosgen stream types and

(2) the consideration of bankfull flow conditions represent a significant advance in culvert

design that is expected to reduce future problems with maintenance and disturbance to the

stream.

13.4.5 Selection of Structure Type (Culvert vs Bridge)

Following completion of the studies discussed above, develop alternative design concepts for

further evaluation for both a culvert installation and a bridge unless the choice of the appropriate

structure to meet site conditions is obvious (extensions of existing structures, magnitude of flood

discharge, etc).

Table 2 below provides a comparison of the advantages and disadvantages of bridges and

culverts.

TABLE 2

BRIDGE vs CULVERT

Part 1 - BridgesAdvantages Disadvantages

Less susceptible to clogging with drift, ice and

debris

Requires more structural maintenance and

rehabilitation over design life than do culverts

Waterway area increases with rising stage of

stream until water begins to submerge

superstructure

Spillthrough slopes susceptible to erosion

damage from highway and bridge runoff

Scour increases waterway opening, reducing

backwater.

Piers and abutments susceptible to failure from

scour

Roadway widening does not usually affect Susceptible to ice and frost formation on deck


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hydraulic capacity

13.4 Location StudiesLess of an impact on aquatic environment and

wetlands when natural channel/flood plain

relationships maintained.

Bridge railing and parapets hazardous to

motorists as compared to recovery areas

generally provided by culverts

Easier to maintain the natural stream channelwidth/depth ratios under the bridge.

Deck drainage may pose a hazard to motorists;scuppers may require frequent clean out

Easier to maintain bedload (coarse sediment)

transport.

Buoyant, drag and impact forces are hazards to

bridges

Easier to maintain passage of fish and wildlife. Susceptible to damage from stream meander

migration

Channel under bridge can enhance habitat via

scour holes at piers and abutments, etc.

Channel under bridge may get more sunlight

Part 2 Culverts

Advantages Disadvantages

Provides an uninterrupted view of

the road.

Silting in one or more cells of multiple cell installations

and/or accumulation of debris and ice can be a problem

requiring frequent maintenance(These problem can be

reduced through the careful application of the criteria in

this design guide)

Roadway recovery area can be

provided

Waterway area is fixed, once water rises above the soffit of

the culvert, until overtopping occurs. May discharge flood

flows to downstream channel at a higher velocity than

would occur in the natural channel

Grade raises and widening projects

can sometimes be accommodated

by extending culvert ends

Roadway susceptible to overtopping and possible

breaching of embankment if culvert clogs with drift, ice or

debris

Most culverts require very little, if

any, structural maintenance

Possible barrier to fish and wildlife passage due to high

velocities, inadequate low flow depths, blocking of

sunlight, etc.

Frost and ice usually do not form on

the traveled way before other areas

experience the same problem

Susceptible to erosion of fill slopes and scour at outlets due

to high exit velocities; may require energy dissipation at

outletsCapacity increases with stage, and

can be increased with improved

inlets

Susceptible to damage from abrasion or corrosion;

Susceptible to failure by piping and/or infiltration

Usually quicker and easier to build Culvert extensions may reduce culvert capacity

Scour is localized, more predictable

and easier to control

Inlets of flexible culverts susceptible to failure by

buoyancy; rigid culverts susceptible to failure from


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separation of joints

13.4 Location Studies/13.5 Culvert Design ProceduresCan be used to control headcutting

or other vertical channel

instabilities

Backwater caused by culvert may cause silting of upstream

channel.

Storage can be used to reduce peakdischarges in small channels (at

interchanges, etc.) for storm water

management.

Loss of sunlight and changed flow conditions cansignificantly reduce viability of stream for habitat within

the limits of the culvert.

May require stream diversions during construction

May cause degradation of downstream channel through

redistribution, collection and concentration of flood flows

at culvert outlet.

13.5.1 Introduction

The Office of Bridge Development is involved with structures on streams having a drainage area

of one square mile or more. Culvert designs for such locations often involve multi-cell boxes or

multiple pipe installations. The concepts presented below represent approaches to achieving the

design objectives for Type B, C, and E streams as defined by Rosgen (Reference 7). Special

considerations for other stream types are discussed in Section 13.6. Section 13.5 generally

applicable to culverts located on a riffle (or step or rapid for Type B streams) and on

comparatively straight channel reaches. Additional considerations regarding the special

problems involved in locating culverts on bends, confluences and in pools are also discussed in

Section 13.6.

The design process involves the following considerations:

design of the main channel culvert to accommodate bankfull flow with minimum change inthe hydraulic characteristics of unit discharge, width, depth and velocity,

design of an upstream transition to the culvert entrance to achieve, within practical limits, astreamlined continuity of the flow and to maintain sediment transport characteristics of

velocity and shear so as to avoid deposition of material or scouring,

design of the culvert installation, including additional flood plain culverts as appropriate, toaccommodate the design discharge in the channel and on the flood plain, and

design of the main channel culvert outlet to minimize impacts to the downstream channel andto stabilize flow conditions for passage of fish.

There is no single standard design method applicable to all streams. The discussion below

should be considered as a guide for addressing the various objectives to be achieved in the

culvert design process, consistent with the site conditions.


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13.5 Culvert Design Procedures_______________________________

13.5.2 Culvert Alternatives

There are many culvert types and combinations of culvert types currently available forinstallation at a stream crossing. A traditional engineering approach for culvert selection has

been to specify the design requirements for acceptable alternatives. This allows the contractor to

select from among approved alternative designs on the basis of cost. This approach works best

for small pipe culvert installations. The selection process becomes more complex as the size of

the stream increases due to the increasing number of factors to consider and the individual

performance characteristics of different types of culverts. Because of this complexity, the SHA

Office of Bridge Development will normally specify the culvert type and size to be installed.

SHA typically provides detailed instructions with regard to the installation procedures to be

followed for its construction.

13.5.3 Water Surface Profiles

Using information obtained from the field and office preliminary studies, utilize the HEC-RAS

model to develop water surface profiles for the following discharges for pre-construction

conditions:

Recurrence Interval Existing Development

Conditions in the Watershed

Conditions of Ultimate

Development in the

Watershed

Bankfull stage X

Q2 X X

Q10 X X

Q design X X

Q100 X X

Q500 *

(bottomless culverts)

X

* The 500-year flood may need to be analyzed for culverts founded on footings to ensure that

the structure is not vulnerable to damage from scour.

13.5.4 Upstream Transition Section

Concept

The purpose of the transition section is to provide a channel section where the flow can transition

smoothly from the stream channel to the culvert entrance. A properly designed transition section

is important in achieving various objectives of the culvert installation:


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Maintaining the natural stream channel stability of dimension, pattern and profile so that overtime channel features are maintained and the channel neither aggrades nor degrades,


Minimizing scour, erosion or deposition at the culvert inlet and in the culvert barrel, Maintaining or enhancing fish and wildlife passage and habitat, and Creating an aesthetic designAdditional right-of-way may be required to properly design the transition. This need must be

recognized early in the project development stage and coordinated with the Office of Real

Estate. When additional ROW will be required for channel transitions, early contact should be

made with affected property owners to explain the nature of the proposed work and how such

work will enhance the stream quality in the transition section (See Section 13.6).

Work in the channel may be necessary to modify the channel cross-section and slope. This need

must be recognized and agreed to early in the project development stage by personnel from other

agencies who will review the project plans. Stream modifications can serve to provide an

attractive design that enhances habitat.

Design Procedure

From the stream stability study, note the stream type of the approach channel and obtain data on

the bankfull depth, width, velocity and slope. The recommended general approach is to design

the culvert width to match the (stable) bankfull width of the upstream approach channel. For B,

C and E type channels, this approach will tend to stabilize the channel since the culvert invert

will serve to control the upstream channel elevation. In such cases, there may not be a need for

much of a transition section between the culvert and the upstream channel other than to minimize

any abrupt changes in the channel elevation between the channel thalweg and the depressed

culvert invert. Some adjustments to the channel banks are normally required to construct the

culvert and its wingwalls or headwalls. Riprap is commonly provided in the immediate vicinity

of the culvert to transition to the channel banks. In some cases, such as for sand or silt channels,

it may be desirable to provide riprap on the channel bed in the transition to the depressed culvert

invert to limit any upstream effect of the culvert on the approach channel or adjacent wetlands.

These upstream transitions are generally short, being on the order of 25 feet or less.

For a Type A, D, DA, F or G channel, there is likely to be special site conditions that will need

to be taken into consideration in the design of the approach section as well as in the design of the

culvert itself (See section 13.6).

There will be situations where there is inadequate ROW available to construct much of a

transition section. In such cases, the culvert should be sized so that the depth and velocity at the

culvert entrance for bankfull flow should approximate the depth and velocity of the flow in the


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approach channel. This will help to maintain the natural flow conditions at the bankfull

discharge and provide for a short but functional transition section (Figure 2).


The design of the main channel culvert and the flood plain culverts are described in the

following sections. The design of the transition section and the culvert installation should bedone concurrently.

13.5.5 Main Channel Culvert

Select a trial size of the main channel culvert:

The culvert width should be about the same as the bankfull width for the reference section.Where practicable, accommodate the bankfull flow in a single pipe (up to 15 to 16 span) or

a single box culvert cell (up to a 20 span). If the bankfull width is too great to carry in a

single cell, provide a multi-cell box or pipe installation that minimizes disruption of bankfull

flow. Box culverts have an advantage over pipe installations in this regard since the spacing

between the box culvert cells is kept to a minimum (Figure 3). In cases where a two cell boxis placed in the channel, consider the use of a W weir upstream of the culvert entrance

(Figure 4). Some modification of the standard weir dimensions may be needed to fit the

culvert site. The weir serves to divide the flow into two thalwegs while increasing the

approach velocity to the culvert to accomplish the following:

- reduced bar deposition and/or bank scour,- increased competence for coarse bed transport,- reduced debris accumulation at the center wall

Where provision is to be made for fish passage, the main channel culvert(s) should bedepressed a minimum of twenty percent below the existing channel bed. Normally, where

this culvert depression is used, the stream will adjust its thalweg and cross-section so as toselect one of the cells for low flows. This will minimize problems in accommodating fish

passage.

Use HEC-RAS to run the water surface profile for the bankfull discharge; adjust culvertslope, type, roughness and dimensions by a trial and error process to maintain continuity, to

the extent practicable, of bankfull flow widths, depths and velocities upstream of, through

and downstream of the culvert. Plot the bankfull depths along the reach of the stream in

which the culvert is located. Adjust the invert elevations to maintain the selected depression

depth in the culvert.

- From the stream stability study, check the width/depth ratios and the stream type forthe reference reach for agreement with the values at the culvert site.

- assume that the depressed culvert section will fill in naturally so that the channelbed in the culvert will be continuous with the upstream and downstream channel

elevations for the bankfull condition (This assumption will need to be reviewed


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using the procedures in Section 13.8). Filling of the culvert depression with stream

bed material during construction should not be necessary. A composite n value


- should be used for the culvert, based on the stream bed material and the culvert

material above the stream bed.

`- since the culvert geometry and roughness will differ from the stream channel, it is

unlikely that any culvert selection will be able to match exactly the

conveyance properties of the existing channel. The objective should be

to design the culvert

system so as to match, to the extent practicable, the existing water surface

elevations for the bankfull flow upstream and downstream of the culvert as

indicated in Figure 1.

- assume no scour hole occurs at the culvert outlet for the HEC-RAS run, unless the

evaluation involves an existing culvert with an existing significant outlet scourhole such as a blow hole. This assumption will need to be checked later on in

the design process.

where the continuity of bankfull flow conditions can be maintained through the culvert, theforegoing design procedure can be expected to provide reasonable assurance of fish passage

for low flows. However, (1) if bankfull flow velocities in the culvert are significantly higher

than in the adjacent channel or (2) if it appears that channel bed load will be swept out of the

culvert for bankfull flow, design modifications (changes in culvert type or slope, addition of

baffles, downstream grade control structures, etc.) may be required and should be

investigated as discussed in Sections 13.8 and 13.9

13.5.6 Flood Plain Culverts

The culvert is a cost-effective hydraulic structure since it serves to collect flood flows upstream

of the highway crossing from the channel and flood plains, convey the flow under the highway

and discharge it into the downstream channel. However, this action of collecting the upstream

flow and discharging it in a concentrated jet into the downstream channel can have an effect on

the stream morphology. A small culvert that severely constricts the flow can initiate degradation

and lead to the creation of an unstable channel downstream of the highway.

One way to minimize this effect is to install additional culverts on the flood plain to convey the

flood plain flows from one side of the highway to the other and thereby reduce the collection andconcentration of flow by the main channel culvert into the downstream channel.

An alternative approach would be to design a bridge for the crossing. However, the costs of

such a solution may be high in comparison with the benefits to be obtained. Stream channels are

dynamic and can adjust over time to changing conditions brought about by a highway culvert

installation when careful attention is given to the stream morphology in the design of the


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structure. The engineer needs to determine the best method of accommodating the flood flows

for the particular site conditions.


The flood plain culverts may also serve to accommodate passage of wildlife. The need to

provide for such passage, and the clearance or size requirements to encourage wildlife to use theculverts should be pursued with environmental specialists during the location/environmental

phase of project development.

The following guidance is provided with regard to the design of the flood plain culverts. The

guidance is based on the design for a box culvert, but a similar approach can be used for other

culvert types and shapes as well.

Location

Where practicable, the flood plain culvert should be positioned on the flood plain well beyond

the channel banks (Figure 5). This location avoids the higher velocity and boundary shear stressin the near bank region of the flood plain, and moves the culvert away from the area of

convergence of the flood plain flow into the culvert. (However, SHA has successfully

constructed flood plain culverts immediately adjacent to the main channel culvert). It also

minimizes the chance for clogging with debris that is carried in the main channel. The upstream

flood plain culvert invert is to be set at the water surface elevation of the bankfull flow. On wide

flood plains, flood plain culverts should be used on both sides of the channel. If the flood plain

is constricted, place the culvert(s) on the inside of the channel bend to avoid convergence and

high velocity flows on the outside of the bend (See also Section 13.9).

Size

The culvert installation is to be sized to meet the state flood plain requirements. Since the flood

plain culvert(s) and the main channel culvert will have different invert elevations, the dimensions

of the various culverts must be determined by a trial and error solution.

Main channel culvert: the culvert width is determined by the procedure discussed in Section

13.4.5 to match the bankfull width; the height of the culvert needs to be established as described

below.

Flood plain culvert: both the width and height of the culvert need to be determined by the

procedure described below.

Appendix A discusses hydraulic design procedures for sizing the flood plain culvert(s) and the

main channel culvert. Culvert design software packages such as the FHWA HY-8 program or

the Corps of Engineers HEC-RAS program can be used to select the culvert sizes (See

References). A primary goal is to design the main channel culvert installation to convey the

main channel flows and the flood plain culvert installation(s) to convey the flood plain flows.


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Judgment must be used to determine the best way to handle the flow for a specific location and

flood discharge.


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The following criteria apply in establishing culvert size and heights:

Provide for freeboard for bankfull flow (Freeboard should be equal to the thalweg ormaximum depth in the channel for bankfull flow See Figure 6).

For pressure flow conditions, an appropriate ratio should be maintained between the mainchannel culvert and the flood plain culverts. As a first step where conditions warrant, it is

suggested that the soffit or crown of the flood plain and main channel culverts be set at the

same elevation.

Match the existing water surface profile of the existing structure. For a structure on newlocation, meet the flood plain requirements set forth in Chapter 9.

The energy line elevation for the design discharge must not exceed the elevation of theupstream edge of traffic lanes.

A minimum cover between the pavement and the top of the box must be provided asdiscussed in Section 13.10.

13.5.7 Culvert Headwall and Endwall

Culverts ends shall be protected with walls designed to meet the site conditions (straight

headwall or endwall, use of wingwalls, etc.). A cutoff wall at least three feet deep shall be

provided below the culvert invert at the headwall and at the endwall to prevent problems of

undermining, piping, scour and erosion of the culvert bedding material and the culvert fill.

Culvert end sections should be designed as hydraulically efficient openings (Appendix A),

should be structurally designed to resist buoyancy and uplift forces acting on the culvert, and

should match the geometry of the roadway embankment.

One of the most important decisions about this aspect of the design is whether the culvert

installation will share a single headwall and endwall, or whether each culvert will be protected

with its own headwall and endwall. It is important to place the flood plain culverts away from

the active channel to minimize the potential for undermining by degradation or migration of the

main channel into the area of the flood plain culvert. If flood plain culverts can be located away

from the main channel and still serve to effectively convey flood plain flows, then it is

reasonable to provide for separate entrance and outlet protection for the flood plain culvert

(Figure 7). If site conditions necessitate placing the flood plain culverts near the banks of the

main channel, they become more vulnerable to undermining. Under this condition, consider

combined headwalls and endwalls for the main channel and flood plain culverts to protect the

culvert installation at its inlet and outlet. In some cases, the size of such a combined headwall

can become massive and render this concept as impractical (Figure 8).


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Protection of the culvert ends needs careful consideration, taking into account the likelihood of

lateral migration of the upstream channel or degradation and scour of the downstream channel

over the design life of the culvert installation. The stream stability study should be reviewed to

consider channel characteristics such as the meander belt ratio, the stream type and the streamsability to adjust to changing conditions.

13.5.8 Hydraulic Analysis

After selection of trial sizes and arrangements of the culvert installation to best fit the channel

morphology, the culvert installation needs to be analyzed for hydraulic capacity to convey flood

flows so as to meet State requirements (See Chapter 9 and Chapter 13, Appendix A). The design

recurrence interval for various classes of highways is set forth in Chapter 10, Bridges.

Water surface profiles should be prepared using HEC-RAS. Adjustments need to be made in the

hydraulic capacity of the culvert installation until the structure conforms to State requirements.In making these adjustments, it should be kept in mind that the width of the main channel culvert

was selected to accommodate the bankfull flow. Where adjustments in capacity are required,

consider increasing the area of the flood plain culverts or the height of the main channel culvert.

Try to minimize changes to the width of the main channel culvert.

Field checks should be made of the development in the flood plain to assure that the culvert

headwater elevations determined by the hydraulic analysis are consistent with State regulations.

13.5.9 Culvert Outlets

The design of the main channel culvert outlet is a critical aspect of the culvert installation withregard to minimizing the impact of the culvert on the stream. When flood plain culverts are

effectively used, they serve to relieve the main channel culvert of much of the hydraulic load.

This in turn simplifies the problem of energy dissipation at the culvert outlets. Normally, energy

dissipation at the outlets of the flood plain culverts can be handled with riprap pads since the

velocities of flow on the flood plain tend to be low.

A reconnaissance study should be made of the stream channel downstream of the proposed

highway crossing to determine its characteristics and to evaluate its stability. If the downstream

channel is steep or unstable, special considerations may need to be taken in the design of the

culvert outlet. Of particular importance is the occurrence of any downstream nick points or

channel drops which indicate that an instability downstream of the culvert is working backtowards the culvert outlet. If the channel reach in the vicinity of the culvert is unstable, an early

decision will need to be made as to whether restoration efforts to stabilize the stream are

necessary. If so, such restoration measures should be designed and constructed as a part of the

culvert installation (See Section 13.6).



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Culvert outlet protection needs to be tailored for the specific conditions at the site, and no single

standard method will be appropriate for all locations. This guide provides an overview of

factors to consider in the design and provides for several examples to illustrate how the outlet

protection can be designed to fit the site conditions.

Case 1 Outlets

Case 1 outlets are defined in this chapter as outlets into a downstream channel and flood plain

that is stable and has a relatively flat slope such as a C-Type or E-Type stream (See Figure 1).

The design concept of the flow distribution for the Case 1 culvert installation is to pass the flow

through the culverts in approximately the same manner as the flow is distributed in the upstream

channel and flood plain. That is, maintain the flood plain flow on the flood plain and pass the

main channel flow in the main channel culvert. Select a design that minimizes changes to the

velocity of flow at the culvert outlets as compared to the velocity of flow approaching the

culverts.

For this case, the flood plain culverts will often outlet onto an essentially flat flood plain. A

riprap pad can be placed at the flood plain culvert outlets to protect the outlets and to minimize

any local erosion on the flood plain. Another approach is to provide a riprap basin at the outlet

with a pool depth designed for energy dissipation. Basins serve to dissipate energy and to

redirect the outlet flow so that it spreads out and redistributes itself onto the downstream flood

plain. In some developed areas, however, pools may represent a nuisance and safety hazard.

Velocities in the main channel culvert for Case 1 culverts can be expected to be higher than in

the flood plain culverts, and are likely to be higher than in the upstream approach channel for the

design discharge. In addition, the downstream channel banks and bed are likely to be vulnerableto scour and erosion. There are a number of alternative designs that can be considered in

providing the desired protection for this case.

A riprap basin at the culvert outlet is a natural and efficient means of dissipating energy. The

basin can also serve to facilitate fish passage from the downstream basin into the culvert barrel.

The outlet basin is particularly effective for stable streams with flat slopes such as Type C and E

streams.

The outlet basin can be pre-formed and protected with riprap to minimize future changes due to

outlet scour. Alternately, a riprap blanket can be provided at the main channel culvert so that the

culvert will carve the scour hole and distribute the riprap within the hole during the occurrenceof future floods. For some situations, such as accommodating passage of resident or non-

migratory fish species in shallow channels, the riprap blanket on the channel bed serves to

facilitate the transition for the fish from the downstream channel to the culvert (Figure 9). These

species of


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fish (chubs, minnows, etc.) are not strong swimmers and cannot traverse culverts with high

outlet velocities.

Where appropriate, consideration can also be given to designing the culvert outlet as a pool usingthe dimensions of pool width, depth, length and other characteristics obtained from the stream

inventory and classification study.

In some cases, it may be necessary to provide for a grade control on the downstream end of the

basin. The purpose of the control is to maintain the outlet scour pool at an elevation to enable

fish to enter the culvert. This control should also provide a means of fish passage from the

downstream channel into the outlet pool. For some locations on steeper slopes, it may be

necessary to design several step-pool arrangements (Figure 10).

The Rosgen cross vane can be used to advantage in some cases to provide a more positive

control to anchor the downstream end of the outlet basin. These devices work well on Type Band C streams where there may be a need to design more than one outlet pool to accommodate

fish passage.

Case 2 Outlets - Structural Energy Dissipators

There are a number of structural design measures that can be used to dissipate energy where

there is a very high outlet velocity from a culvert. The FHWA publication HEC-14 contains

details regarding outlet protection. Such devices are generally not recommended for use if there

are other design approaches that can be used to reduce the outlet velocity. Alternative

considerations include use of other culvert materials and shapes, modification of culvert slopes

or design of a bridge in lieu of a culvert.


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13.6 Culvert Design Procedures_- Special Design Considerations___________

In some cases, particularly for A, DA, F and G Type streams, stream morphology becomes of

paramount importance to the culvert design, requiring the services of engineers with specialized

experience and knowledge. The discussion below addresses possible considerations affecting

the design of culverts on these streams. The primary consideration for such specialized designproblems should be to obtain the services of engineers with experience in working with these

stream types. As noted elsewhere in this chapter, extensive work outside of the normal highway

rights-of- way represents a departure from traditional design practices. Such specialized designs

will need to be justified on a case by case basis.

Additional right-of-way may be required to modify the stream. This need must be recognized

early in the project development stage to obtain approval of the concept and to coordinate the

work with the Office of Real Estate. When additional ROW will be required for channel

modifications, early contact should be made with affected property owners to explain the nature

of the proposed work and how such work will enhance the stream quality.

Work in the channel may be necessary to modify the channel cross-section and slope. This need

must be recognized and agreed to early in the project development stage by personnel from other

agencies that will review the project plans. Stream modifications can serve to provide an

attractive design that enhances habitat.

13.6.1 Type A Streams (Figure 11)

The A-type stream is a steep, entrenched and confined channel with stream slopes in the range of

4 to 10% or more. In Maryland, this stream type tends to occur mostly in small drainage basins.

The recommended general concept for a highway structure on an A type stream is to span the

entire channel, placing the structure footings beyond the limits of the channel banks. Thisapproach minimizes the effect of the structure on the stream and on bankfull flow conditions.

Use of a bottomless culvert, rigid frame or bridge may avoid the need for any work in the stream

channel, including energy dissipation at the outlet of the structure. Typically, such structures can

be constructed as a single span. If the abutment footings are located within the limits of the 500-

year flood plain, appropriate measures will need to be taken to protect the abutments from

damage from scour. For stream types A3, A4 and A5, particular attention should be given to

evaluating the need for a grade control structure at the culvert outlet. If a culvert installation is

used on a Type A stream, the culvert should be located in the area of steps, rapids or chutes.

13.6.2 Type DA Streams (Figure 12)

The DA stream types are highly interconnected channel systems developing in gentle relief

terrain areas exhibiting wetland environments with stable channel conditions. The stream

channels have highly variable width to depth ratios, flat slopes (< 0.005), low bedload and stable

banks.

13.6 Culvert Design Procedures_- Special Design Considerations__________


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In Maryland, such streams are encountered mostly in the coastal physiographic regions. The

recommended general concept for a highway structure on such streams is to maintain the

existing channel system to the extent feasible and to locate the culverts in convergence regions.

This can be accomplished by providing:

a combination of bridges and culverts at the crossing (preferred approach), culverts to carry each of the individual stream channels under the highway, or a bridge to span the entire channel system.13.6.3 Type F Streams (Figure 13)

The F stream type is an entrenched, meandering riffle/pool channel on a low gradient with a high

width to depth ratio. Type F streams tend to be laterally unstable with high bank erosion rates.

Studies by Rosgen have shown that the unstable nature of F type streams can lead to a

reestablishment of a functional flood plain inside the confines of a channel that is consistently

increasing its width within the valley. Because of the potential for the lateral instability of the F

channel, an investigation should be made of the stream reach upstream and downstream of the

proposed highway crossing. Consideration needs to be given to the potential for a continuingevolutionary cycle of the F channel with accompanying downcutting of the channel thalweg and

building of a new flood plain level. Design alternatives for a highway stream crossing include:

1. A stream rehabilitation project to modify the Type F channel (Figure 14) to a more stablechannel form such as a Type C channel. While this approach may require additional ROW

purchases or easements, it may also achieve offsetting cost savings in the form of a smaller

main channel culvert or bridge (to match the lower width to depth ratio of the re-established

channel). Another anticipated benefit would be lower maintenance costs associated with a

stable channel. The re-established channel will be more efficient in transporting sediment,

and can be designed to accommodate fish passage in the reach of the stream affected by the

highway crossing.

2. A bridge to span the main channel with abutments set back from the channel banks and, ifnecessary, placement of circular piers in the channel. A clear span without piers is preferred,

since further downcutting and shifting of the channel may increase the scour potential at the

piers. If the bridge width provided is of adequate length, the stream may eventually re-

establish a second stage channel within the limits of the original F channel.

3. A culvert installation. The most feasible approach would be to consider the evolutionarycycle of the F channel and to design the installation with a main channel culvert (to match

the lower width to depth ratio of the re-established channel) and with flood plain culverts

(Alternative 1 above). If this approach cannot be worked out, a wide culvert should be

13.6 Culvert Design Procedures_- Special Design Considerations__________


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considered, having a width equal to or greater than the bankfull width of the F channel. The

culvert should be of adequate size to convey the design flow assuming that an evolutionary

channel will form inside the F channel and culvert. The geometry and arrangement of the

culvert cells should anticipate bankfull flow in a second stage channel so that one or more of

the cells can function as a future main channel culvert. Stabilization of the steep F channel

banks may be needed to protect against further lateral movement of the channel that wouldresult in outflanking of the culvert and subsequent erosion of the highway embankment

(Figure 15).

13.6.4 Type G Streams

The G or gully stream type is an entrenched narrow and deep step/pool channel with a low to

moderate sinuosity. Channel slopes generally range from 2% to 4% although G channels may

be associated with gentler slopes where they occur as down-cut gullies in meadows. With the

exception of channels containing bedrock and boulders, the G stream types have very high

bank erosion rates and a high sediment supply. Channel degradation and sideslope rejuvenation

processes are typical.

A culvert design on a G type stream should be approached with caution in recognition of the

potential problems with bank erosion and sediment discharge. A bridge may be a better

alternative in some cases and it is recommended that a bridge alternative always be evaluated for

G-3 to G-6 Type streams.

Prior to selecting a culvert installation for a G type stream, a detailed study should be

conducted of the stream reach to determine whether work is necessary to stabilize the channel

upstream and downstream of the culvert.

The upstream approach channel may require stabilization in order to control bank erosionand the development and movement of meanders towards the highway. Withoutstabilization, the stream may outflank the culvert and attack the highway embankment.

Stabilization efforts may include consideration of the conversion of the G channel to a B

type channel. This is done by increasing the width to depth ratio as well as the entrenchment

ratio (Figure 16).

Upstream stabilization may require a right-of-way easement including an access road(beyond the channel banks) to maintain the channel stabilization measures.

The design of the upstream transition section should be made with careful attention to theenergy slope of the flow to minimize any deposition of bed load at the culvert entrance.

The depth and velocity of flow for bankfull flow conditions in the culvert should beconsistent with depth and flow velocity in the upstream (stabilized) channel. Accordingly,



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culvert longitudinal slopes should be similar to the upstream channel slopes. Any significant

flattening of the culvert slope or the energy line of the flow through or upstream of the

culvert may result in silting up of the culvert barrel or the approach transition section.

Culvert outlet velocities should be consistent with existing flow velocities in the channel tothe extent feasible. Significant increases in outlet velocities coupled with the formation of anoutlet jet into the channel can create problems with downstream channel instability. This can

result in channel degradation and headcutting leading to the undermining of the outlet and

possible failure of the entire culvert installation.

Stabilization of a downstream channel reach may be necessary to control degradation on G-3to G-6 streams. Stabilization efforts may be required beyond the highway right-of-way

limits. In some cases, the construction of two or three step/pools can serve effectively to

stabilize the downstream channel.

In general, the stability problems with G type channels become more difficult as the culvert

length increases. A bridge alternative has the advantage of spanning the stream entirely, thereby

avoiding changes to the stream regime. Bridge abutments should be placed well back of the

channel banks taking into account the potential for future channel widening and an evolution to

an F (Gc F) or B (G B) type stream (Figure `17).

13.6.5 Stream Crossings at Bends, Pools and ConfluencesThe foregoing discussion regarding the location and design of culverts is based on the

assumption of a favorable crossing location of the stream such as a straight reach on a riffle. In

some cases, however, it may be necessary to construct a culvert in a bend or a pool or to dealwith a stream confluence. These situations introduce a number of other considerations in the

design process.

Bends (Figure 18)

Bend locations can present a number of challenges to a culvert design. It is important to assure

that the reach upstream and downstream of the bend, as well as the bend itself, be reasonably

stable. If it is not, consider designing upstream or downstream transition sections to reestablish a

stable channel. Align the culvert entrance and outlet with the stream channel to provide for a

streamlined flow pattern and accompanying efficiency in the conveyance of sediment and flood

flows. To achieve the design objectives in bends, it may become necessary to adjust the locationof the channel in the vicinity of the culvert inlet or outlet. Such adjustments should be made so

as to maintain, to the extent practicable, the existing pattern, profile and dimension of the stream

and avoid straightening of the channel.



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Because of the additional problems and costs associated with designing a large culvert with a

radius of curvature, this approach is not generally recommended. In some cases, however, use of

a curved culvert may serve to minimize problems associated with disruption of the existing

channel.

Pools (Figure 19)

When a culvert is to be located in an existing pool, the culvert design should be directed at

maintaining the characteristics of the pool. The geometry of the pool can be obtained from the

stream inventory data. The culvert width should be based on the average bankfull width in the

pool. The bankfull depth will be much greater in the pool than it is for a riffle section. Fish

passage is facilitated in a pool and the culvert invert may not need to be depressed below the

stream thalweg in the pool. If the culvert extends beyond the pool into the downstream riffle, it

may remove the existing control, which established the glide and the run at the end of the pool.

To the extent practicable, the pool depth should be extended into the downstream riffle beyond

the outlet area of the culvert a sufficient distance in order to reestablish a control for the glide

and the run.

Pools and riffles tend to move downstream over time. In the event that an upstream riffle moves

down into the culvert, consider a design that will still provide for sufficient flow depth for fish

passage. Downstream grade control structures can be used to maintain the pool elevation.

Confluences

A recurring problem in culvert design is the treatment of a stream confluence that lies within the

limits of the proposed highway alignment. The general rule for such cases is to maintain

separately the natural flow pattern of each stream channel even if this involves construction of

two culverts. (Bridging of the confluence is notgenerally recommended because of problemswith scour, erosion or sedimentation.) The site conditions for each location will need to be

evaluated to determine the best location of the confluence relative to the highway.

Relocating the stream confluence immediately upstream of the highway culvert may create

future problems with sediment deposition at the confluence and culvert entrance. It is usually

preferable to relocate the stream confluence downstream of the highway crossing, especially if

the streams are different sizes and types (Figure 20). For example, a small tributary G Type

stream emptying into a larger E Type stream is liable to carry a significant sediment load that

will be deposited at the confluence and the culvert entrance. Moving the confluence downstream

of the highway will require the construction of two separate main channel culverts. Where the

two streams share a common flood plain, the hydraulic design will need to be developedconsidering the total flow in the flood plain system and the characteristics of the hydrographs

(time to peak, etc.) of the two streams.



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If both tributaries are of the same type and approximate size, it may be feasible to locate the

confluence upstream from the culvert, provided that sediment transport can be maintained

through the culvert (Figure 21). For this case, lateral stabilization of the channel banks may be

needed upstream of the culvert.


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13.7 Applications Case Histories

The guidelines and criteria in this chapter provide an overview of the concepts to be applied in

the design of a culvert installation in order to meet the design objectives. Since each stream

crossing is unique, having its own set of conditions and considerations, it is not feasible to

provide a step by step process that would serve equally well for every culvert location.However, the SHA has now constructed several culvert installations that have been designed

utilizing the Rosgen stream classification system and the basic concepts presented in this

chapter. Designers are encouraged to review the hydraulic reports for these culvert installations

to note how the environmental, morphological, hydraulic and hydrologic conditions were

evaluated and taken into account in the culvert design.

These culvert installations include:

1. Route 50 Salisbury Bypass over Wicomico River (combined bridge and culvert installationfor a Rosgen DA stream type,

2. Route 25 over Beaverdam Run, Baltimore County (Rosgen C4 Channel),3. Route 382 over Full Mill Branch and Spice Creek, Prince Georges County (Rosgen E6

channel),

4. Arena Drive over Tributary to Southwest Branch, Prince Georges County (Rosgen G4channel),

5. Route 191 over Tributary to Cabin John, Montgomery County (Rosgen C3/B3 Channel)


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Section 13.5, Culvert Design Procedures, presents general recommendations for the sizing and

hydraulic analysis of culvert installations and their outlets. The procedure outlined in Section

13.5 serves to produce a reasonable estimate of the culvert outlet velocity for a given discharge.

Where adequately sized flood plain culverts are provided, the culvert outlet velocity in the main

channel should be reasonably consistent with channel velocities for existing conditions. For

culverts on mild slopes, modest increases in velocities of less than 20 percent normally will be

dissipated by the formation of the outlet scour hole. Culverts on steep slopes, however, can

represent more of a problem. This section expands on the guidance in Section 13.5 for the

condition where there is a need to study the scour potential in greater detail.

Evaluation of three flows is recommended: bankfull flow, design flow, and incipient overtopping

flow, within practical limits. Information required for the analysis includes:

Information on the stream channel, its slope and composition of channel bars as determinedfrom the stream inventory and classification (As noted in Section 13.5, particular attention

needs to be given to the stability of the downstream channel section and any tendency

towards degradation of the channel bed in this section.)

Composition of the bed material in the outlet channel, Flow velocity, slope and depth at the culvert outlet and at various locations in the

downstream channel.

Bankfull Flow Conditions

Compute the Mannings n value (bankfull flow) and critical shear stress for the bed material

using the information presented in References 7 and 8, Shields criteria or other references.

Estimate the size of bedload being transported at or near the bankfull stage by sampling nearby

channel bars. Compute the shear stress at several locations downstream of the culvert outlet

( = RS) where the channel is stable. By a comparison of existing and proposed conditions

determine the zone of influence in the downstream channel where velocities and shear stresses

of proposed conditions are higher than for existing conditions.

Based on the above information, determine whether outlet flow conditions are acceptable. If not,

consider either (1) redesign of the culvert or (2) use of one or more riprap basins with

accompanying grade control structures in the downstream channel to dissipate energy. These

pools may require construction of a step-pool type of control using rock weirs, with careful

attention given to passage of fish at the weirs (Figure 22).


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Where significant problems are encountered with regard to the effect of high outlet velocities on

the stability of a downstream channel, it may be prudent to evaluate an alternative design rather

than to acquire easements and to construct elaborate outfall structures.

Design Flow Condition

Any culvert outlet control feature provided for bankfull flow will need to be evaluated for the

design flow to assure that it will not fail and endanger the stability of the culvert. The evaluation

process is similar to that used for bankfull flow. However, the estimation of Mannings n may

need to be modified to account for the effect of vegetation and other conditions of flood flows. It

is essential that the culvert and its outlet protection system remain stable for conditions of the

design flow.

Incipient Overtopping Condition

In some cases, the incipient overtopping condition may be the design condition. In other cases,

this condition may occur only for a very rare flood well in excess of a 100-year or 500-year

flood. The probability of occurrence needs to be given primary consideration in the evaluation of

this flood event. The incipient overtopping condition is used as a worst case scenario for

evaluating culvert outlet conditions. If this flow condition will create catastrophic results due to

extreme outlet velocities, a reevaluation of the culvert design is warranted.


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13.9 Silting of Culverts

Main Channel Culverts

The design procedure in Section 13.5 is based on the assumptions that:

the culvert is aligned with the upstream channel, the upstream channel is stable and is neither aggrading or degrading, the two foot depressed culvert invert will remain filled with bed load material from the

upstream channel, and

silting of the main channel culvert installation above the two-foot depression will beunlikely if the culvert is designed to maintain the dimension, pattern and profile of the stable

channel morphology associated with the bankfull flow.

An important objective of the stream inventory and classification study, Section 13.4.4, is to

determine the site conditions and to identify potential problems with sedimentation (stream bed

aggradation or degradation, bank erosion, etc.). There will be various situations where the

above assumptions will not apply. Two examples are discussed below:

Example One (Figure 23) involves flow through a box culvert on a steep slope where the

Mannings n value in the culvert is lower than in the approach channel. In this case, there is a

concern that the bed material may be swept out of the culvert, resulting in a lower n value and

a resultant high culvert velocity. If fish passage is a design objective for this culvert, special

features may need to be incorporated in the design of the culvert barrel to reduce culvert

velocities and to provide resting places for fish. (Reference)

Example Two (Figure 24) involves a location where a steeper approach channel flattens out just

upstream of the culvert. This is not a desirable culvert location; yet, constraints sometimes

require that a culvert be located in a less than ideal stream reach. In this instance, there is a

concern that the culvert will be unable to convey the bed load from the upstream channel and

will silt up.

Few procedures are available for estimating sediment conveyance capacity and deposition

potential in culverts. One procedure that is currently under review and evaluation by the

Maryland SHA is presented in a paper written by Dennis Richards and Michael Zeller of Simons

Li and Associates. This paper is included in Appendix B of this Chapter.


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The following approach is recommended in the use of the Appendix B procedure:

1. Use Method B, carefully following the steps outlined by the authors in determining theparameters contained in the Method B Equation.

2. Compute Rd on the basis that the bed load will fill in the depressed culvert section, and thatthe channel bed slope will be essentially the same from the outlet through the culvert barrel

and up to the approach channel. This will usually involve the computation of a composite

roughness n value in the culvert for the bankfull flow.

3. Calculate Rs the bed-material sediment-transport ratio, channel to culvertIf Rs is less than 0.8, the culvert will most likely be able to transport the sediment being

delivered by the approach channel. If the value of Rs is greater than 1.0, sedimentation may

occur and design modifications should be considered.

Using the same reasoning, the smaller the value of R s, the more likely the chance that the bed

load will not remain in the depressed culvert bottom for the bankfull flow. This can be checked

by using the procedure discussed in Section 13.7 to see if the shear stress in the culvert exceeds

the critical shear stress of the bed load for the bankfull condition. If this is the case, velocities

for the bankfull condition should be based on using the n value of the culvert material without

consideration of the roughness of the bed material.

Rosgen has pointed out (Reference 13) that the Appendix B method may be helpful for

preliminary analysis, but may not always accurately reflect sediment transport processes for

some stream types. For large culverts or for environmentally sensitive sites, he proposes the useof the method outlined below for gravel streams:

Compute the dimensionless shear stress for a stable (reference) reach downstream from the

culvert under study.

Set this value equal to the Shields parameter and compute the largest particle that is expected to

be moved for conditions of the bankfull flow. Verify that this size particle is being moved in the

reference reach by comparing the calculated size with the largest size of particle found in the

lower third of a point bar in the reference reach.

Determine bankfull flow conditions in the proposed culvert and compute the Shields parameterfor these conditions leaving the particle size as an unknown. Set the Shields parameter equal to

the dimensionless shear stress computed in step 1 above and solve for the largest particle size

that will be moving in the culvert.



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Compare the computed particle size from step 3 with the largest particle size measured and

computed for the reference reach. If the culvert can pass a particle larger that the particle size in

the reference reach, the culvert should not silt up. If the particle size passed by the culvert is

much larger than the particle size in the reference reach, it is an indication that the culvert may

be self-cleaning of its bed load.

Detailed information regarding these calculations are contained in unpublished training course

material presented by Rosgen to the Maryland SHA (Reference 14)

Flood Plain Culverts

The design procedure in Section 13.5 is based on the concept that (1) the bed load carried by a

stream is concentrated in the main channel and (2) that very little bed load is carried by overbank

flow. This concept is applicable to many locations since the higher roughness values and lower

depths of flow on the flood plain result in shear stress values that are lower than the critical shear

stress of the protective vegetation. There are locations where considerable amounts of bedloadmaterial are conveyed out of the stream bed and deposited on the flood plain or along the

riverbank. These locations typically introduce a major change in the sediment transport

conditions due to the highway crossing or the natural flow conditions in the stream. Placement

of flood plain culverts should be made with this thought in mind. Examples include:

Formation of bars on the inside of bends, Goose neck bends where flood flows and sediment are transported across the flood plain

neck separating the channels,

Stream confluences, Diversion of flood flow due to blockage of culverts or bridges by sediment and debris.

13.10 Effects of Ice and Debris


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Highway culverts should be located so as to avoid areas that have a high potential for blockage

by the deposition of stream sediments and debris. Because of Marylands location and climate,

ice jams have not been a significant problem in the operation of highway culverts. Debris, on

the other hand is a continuing problem, and the consequences of debris accumulation should be

considered in the location and design of the culvert. Wherever practicable, access should be

provided for personnel and equipment to clean the culvert entrance.

Debris is defined as any material moved by a flowing stream. This includes a combination of

floating material, suspended sediment and bed load. A streams propensity for carrying debris is

based upon watershed land uses and its stream and flood plain characteristics. Debris is likely to

be a problem in mountainous or steep regions. The site investigations of a proposed stream

crossing should include evaluation of the following conditions:

Stream velocity, slope and alignment, Presence of eroding banks and the types of trees or shrubs on the banks vulnerable to being

undermined and washed away,

Watershed land uses, particularly logging, cultivation and construction, Watershed response to storms, such as flash flooding in steep terrain, Storage of debris and materials on the flood plain (logs, lumber yards, etc.),Debris can accumulate at a culvert inlet or become lodged in the inlet or barrel. Severe

blockages can lead to the ponding of water upstream of the culvert with damage to developed

properties, overtopping of the roadway and subsequent destruction of the entire culvert

installation.

Some debris accumulation can be anticipated at most culvert installations. Routine maintenance

operations including removal of the debris will serve in most cases to minimize the hazard of

blockages. If the site reconnaissance reveals a significant potential for debris accumulation,consideration should be given to minimizing the problem and its effect on the safe operation of

the highway. Alternative approaches include use of a single cell (vs a multiple-cell culvert

installation) or selection of a bridge. The use of flood plain culverts will serve to provide relief

openings for conveyance of the flood flows in the event the main channel culvert becomes

blocked.

Debris deflectors and control structures have not been used extensively in Maryland, and they

are not generally recommended for inclusion on new construction. Where debris accumulation

has become a continuing maintenance problem and safety hazard at an existing culvert, a debris

control structure or deflector should be considered as one way of lessening the extent of the

problem. Design information for commonly employed debris control structures and deflectorscan be found in the FHWA Publication HEC-9, Debris Control Structures, Reference 5.

13.11 Structural Requirements


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Cast-in-place structures such as box culverts are individually designed for the given site

conditions in accordance with the policies and procedures of the Office of Bridge Development.

Pre-cast boxes, pipes and pipe arch structures are to be designed in accordance with the criteria

specified by the manufacturer, including provisions for minimum cover. The minimum cover for

concrete and metal round pipe is three feet. The manufacturers fill height tables andspecifications should be consulted when determining minimum and maximum cover for other

culvert shapes and materials.

Specialized designs, such as long span culverts, are evaluated on a case by case basis.

An important requirement for metal culvert installations is to assure that the headwalls are

properly anchored to resist buoyancy and up-lift forces.

SHA PPM D78-15 (4) dated July 31, 1990 entitled Length and Treatment of Culverts provides

additional information on the end treatment of culverts. This directive requires that a cutoff wall

with a minimum depth of 3 feet below the culvert invert be provided at headwalls and endwallsto protect the culvert from scour and erosion.

SHA PPM D84-29 (4) dated July 26, 1994 entitled End Treatment for Pipe Culverts also

provides general guidance on the design of culvert end sections.


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13.12 Durability Requirements

The minimum service life for a culvert designed by the Office of Bridge Development is 75

years. On a case by case basis, a longer service life of up to 100 years may be specified where

the construction of a replacement structure would result in extraordinary costs or involve an

extended period of disruption of traffic service. An example of this case would be a culvert undera high fill on a major arterial or Interstate highway.

The design service life of a culvert is defined as the expected maintenance free service life of

each installation.

The Office of Bridge Development has adopted the durability requirements in the Highway

Drainage Manual, Chapter 9, Culverts, for all concrete and metal pipe culverts. For structural

plate pipe, ellipses and arches, the maintenance free service life, with respect to corro

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